Acoustic resonance damping apparatus

ABSTRACT

Apparatus is disclosed which utilizes an acoustic impedance matching technique to substantially reduce the occurrence of acoustic resonances in an electro-optic crystal. In one embodiment, the acoustic resonance damping apparatus is adapted to apply an electric field to the electro-optic crystal. Several additional embodiments are disclosed.

United States Patent [151 3,663,091 [4 1 May 16,1972

Lee

[541 ACOUSTIC RESONANCE DAMPING APPARATUS [72] Inventor: Tzou-Chang Lee,Minneapolis, Minn.

[73] Assignee: Honeywell Inc., Minneapolis, Minn.

[22] Filed: Dec. 17, 1969 21 Appl. No.: 885,795

[52] U.S. Cl ..350/160, 350/150 [51] Int. Cl. ..G02f1/20 [58] FieldofSearch ..350/149, 150, 160, 161; 310/82 [56] References Cited UNITEDSTATES PATENTS 3,365,581 l/1968 Tell et a1 ..350/150 3,403,271 9/1968Lobde11etal.... ..310/8.2

3,454,325 7/1969 Ohm ..350/160 3,492,596 l/l970 Vorie ..350/149Stephany, Piezo-Optic Resonance in Crystals of Dihydrogen PhosphateType," JOSA, Vol, 55, 02, 2/65 pp. 136-142 Primary Examiner-Ronald L.Wibert Assistant Examiner.leff Rothenberg Attorney-Lamont B. Koontz andRobert O. Vidas [57] ABSTRACT Apparatus is disclosed which utilizes anacoustic impedance matching technique to substantially reduce theoccurrence of acoustic resonances in an electro-optic crystal. In oneembodiment, the acoustic resonance damping apparatus is adapted to applyan electric field to the electro-optic crystal. Several additionalembodiments are disclosed.

4 Claims, 6 Drawing Figures 1 ACOUSTIC RESONANCE DAMPING APPARATUSBACKGROUND OF THE INVENTION The present invention relates to anelectro-optic light beam modulator and particularly to apparatus forpreventing the occurrence of acoustic resonances therein.

The electro-optic (E-O) effect exhibited by such crystals as potassiumdyhydrogen phosphate (KDP) and lithium niobate (LiNbO has beenadvantageously utilized to selectively modulate a light beam. Thisutilization has increased greatly since the advent of the laser.However, the electro-optic efiect is inherently accompanied by apiezoelectric effect which is detrimental to the performance of an E-Ocrystal in the following manner. First, the mechanical strain induced bythe piezoelectric effect causes a change in the physical dimensions ofthe crystal resulting in additional modulation of an incident lightbeam. Secondly, the piezoelectric effect gives rise to a photoelasticeffect which results in a change in the crystals refractive index. Thiseffect is commonly referred to as the indirect E-O effect. At certainfrequencies of the modulating field applied to the E-O crystal, thecrystal undergoes mechanical or acoustical resonance. At thesefrequencies the two above-mentioned effects are particularly strong. Asa result, broad band light beam modulation is not obtainable unless thepiezoelectrically induced acoustic vibrations are dampenecl.

The prior art has attempted to suppress the piezoelectrically inducedacoustic vibrations by mechanically clamping the electro-optic element.The mechanical clamping dampens the low frequency acoustic vibrations.In some cases, the apparatus, including the E-O element, is thenimmersed in a viscous fluid which dampens out the higher frequencyacoustic resonances. For a fuller description of such a dampingtechnique see US. Pat. No. 3,454,325 to E. A. Ohm entitled Optical WaveModulator With Suppressed Piezoelectric Resonances.

The prior art damping technique described above necessitatesconsiderably more complex apparatus than the present invention.

SUMMARY OF THE INVENTION The present invention utilizes an acousticvibration damping means positioned contiguous to at least one face of alightmodulating, E-O crystal. The acoustic vibration damping means hassubstantially the same acoustic impedance as he E- crystal so that thepiezoelectrically induced acoustic vibrations incident the interfacebetween the E-O crystal and damping means are transmitted into thedamping means.

Utilizing the present invention, internal reflection of the acousticenergy at interface of the crystal and the surrounding medium isprevented. The acoustic energy transmitted out of the E-O crystal isdampened or directed such that it does not return to the crystal. As aresult, acoustic resonances within the crystal do not occur. Thus,broad-band electro-optic modulation is achieved utilizing relativelysimple apparatus in comparison with the prior art.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates one embodiment ofthe present invention having means for damping acoustic vibrationsoccurring along one dimension within an E-O light beam modulator.

FIG. la is a cross-sectional view of the acoustic vibration dampingmeans illustrated in FIG. 1.

FIG. 2 illustrates another embodiment of the present invention whereinthe acoustic vibration damping means are adapted to apply an electricfield to the B0 light beam modulator.

FIG. 3 is a cross-sectional view of the apparatus shown in FIG. 2.

FIG. 4 is a further embodiment of the present invention wherein theacoustic vibration damping means dampens the acoustic vibrationsoccurring along two dimensions within the E0 light beam modulator.

FIG. 4a is a cross-sectional view of the acoustic vibration dampingmeans illustrated in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The light beammodulator illustrated in FIG. 1 comprises a composite electro-optic(E-O) crystal, generally designated 10, and acoustic vibrations dampingmeans, generally designated 16 and 17. For purposes of thisspecification, the term light beam modulator includes electro-opticelements which provide either light beam deflection or light beammodulation such as is achieved by rotating the polarization vector of anincident light beam. Furthermore, the phrase acoustic resonance damping"refers to a substantial reduction in the amplitude of the resonance anda lowering of the resonance frequency.

As illustrated in FIG. I, electro-optic crystal 10 is composed of E0crystals 11 and 12 which are positioned contiguous one another to form aboundary l3 therebetween. Deflection of an incident light beam isachieved as the beam traverses boundary l3.

Acoustic vibration damping means 16 comprises an acoustic impedancematching means 20 and an acoustic vibration absorbing means 21. Acousticimpedance matching means 20 is positioned contiguous one of theplurality of faces of composite E-O crystal 13. Likewise, acousticimpedance damping means 17 comprises an acoustic impedance matchingmeans 24 and an acoustic vibration absorbing means 25. Acousticimpedance matching means 24 is positioned contiguous a face of compositecrystal 10 oppositely disposed from impedance matching means 20.Acoustic impedance matching means 20 and 24 have acoustic impedanceswhich are substantially identical to the acoustic impedance of E-Ocrystals 11 and 12 respectively. For example, when E-O crystals 11 and12 are composed of lithium niobate (LiNbO one choice of material foracoustic impedance matching means 20 and 24 is also LiNbO By choosingidentical materials, the acoustic impedance is, of course, the same forboth mediums. This being the case, there is no significant acousticenergy reflection at the interface between the two media. Acousticabsorbing means 21 and 25, is for example, a material known as coustibab(style No. CC-488c/AFA) manufactured by Carter Rice Storrs & BementIncorporated.

Electrode means (not shown) for applying an electric field to E-Ocrystals 11 and 12 are separately deposited on each of the oppositelydisposed faces of 1 1 and 12 parallel to the plane defined by the X andY axes. The technique for applying an electric field to an E-O crystalis well-known in the art.

In operation, a light beam is incident composite crystal 10 along the Yaxis. An electric field is applied to at least one of the E-O crystals11 and 12 to obtain an amount of light beam deflection proportional tothe magnitude of the applied field. For a complete description of lightbeam deflection using E-O crystals, see the article entitled Light BeamDeflection with Electro-optic Prisms" by T. C. Lee and J. David Zookappearing in IEEE Journal of Quantum Electronics, Vol. QE-4, No. 7, July1968.

As mentioned previously, the E-O effect is inherantly accompanied by apiezoelectric effect. For the case where E-O crystals 11 and 12 arecomposed of LiNbO the strains piezoelectrically induced within thecrystal are given by SI 2 31 3 3 aa a Where:

S =Induced strain along the X axis;

S,= Induced strain along the Y axis;

8;, Induced strain along the C axis;

E Magnitude of the applied electric field;

d Piezoelectric coefficient along the X axis;

41 Piezoelectric coefiicient along the C axis;

The strains S and S excite longitudinal mode resonances along the X andY axes, respectively. The magnitude of these stains, without an absorbermeans present, is dependent upon the acoustic Q of the -0 crystal. Asillustrated, acoustic impedance matching means and 24 are positionedcontiguous E-O crystals 11 and 12, respectively, so as to absorb theacoustic vibrations generated along the X dimension. By choosing amaterial having substantially the same acoustic impedance as E-Ocrystals 11 and 12, the acoustic energy, illustrated as wavy line 22 inFIG. la, is entirely transmitted across the interface between the E-Ocrystal and the acoustic impedance matching means. The acoustic energytransmitted into acoustic impedance matching means 20 is internallyreflected at face 23 toward acoustic vibration absorbing means 21. Uponincidence on absorbing means 21, the acoustic energy is substantiallyabsorbed. However, a minor portion of the acoustic energy is reflectedat the interface between the acoustic impedance matching means and theacoustic absorbing means. This reflected energy is again incident onface 23. In the embodiment illustrated, the angle a is chosen greaterthan 45 so that the reflected energy incident face 23 is not directedback into the composite crystal 10. Similarly, acoustic energy is alsotransmitted out of composite crystal 10 by dampening means 17.

The strain S excites a thickness-mode extensional resonance along the'Caxis. The damping of this extensional resonance is discussed inconjunction withthe embodiment illustrated in FIGS. 2 and 3.

The acoustic damping means does not, of course, have to be in the formillustrated in FIGS. 1 and 1a. Rather, any configuration can be utilizedso long as it provides substantially entire transmission of the acousticenergy out of composite crystal 10 and attenuates or directs theacoustic energy transmitted out of crystal 10 such that the energy doesnot return to the crystal. Thus, it is not necessary that the acousticdamping means include separate absorber means 21 and 25. As explained inconjunction with the embodiment illustrated in FIGS. 2 and 3, sometimesa properly shaped acoustic impedance matching means is adequate toachieve damping of the acoustic energy.

Shown in FIG. 2 is a composite electro-optic crystal, generallydesignated 30, which is formed by a plurality of adjacent polyhedrons31a, 31b and 31c. A cross-section along the line 3-3 is shown in FIG. 3.In the embodiment illustrated, polyhedrons 31a and 310 are in the formof substantially identical right triangular prisms and polyhedron 31b isin the form of a substantially isosceles triangular prism. E-O crystals31a, 31b and 310 are positioned contiguous one another to form thecomposite E-O crystal generally designated 30. Composite crystal 30 isin the form of a rectangular parallelepiped. As illustrated, each of thecrystals 31a, 31b and 31c is composed of LiNbO and therefore each has anelectric field dependent index of refraction. To maximize light beamdeflection, E-O crystals 31a, 31b and 31c have electric fields along theC-axis in opposite directions as illustrated by arrows 40a, 40b and 400.

Acoustic vibration damping means 34a and 34b and acoustic vibrationdamping means 36a and 36b are positioned contiguous oppositely disposedparallel faces of right triangular prisms 31a and 310, respectively.Similarly, acoustic vibration damping means 35a and 35b are positionedcontiguous oppositely disposed parallel faces of isosceles triangularprism 31b. As illustrated, the damping means 34a, 34b, 35a, 35b, 36a and36b are pyramidal in shape. The pyramidal shape of the acousticvibrations damping means causes multiple internal reflections of theacoustic energy within the damping means which results in the acousticpath being significantly lengthened thereby (l) lowering the acousticresonance and (2) substantially reducing the magnitude of the resonance.Thus, the thickness-mode extensional resonances caused by the strain 5are suppressed by acoustic damping means 34a, 34b, 35a, 35b, 36a and36b.

For the case in which crystals 31a, 31b and 310 are composed of LiNbOthe acoustic vibration damping means can be composed of copper. Sincelithium niobate has an acoustic Ala impedance along the C-axis of 3 l0glcm -sec as compared to that of 4 X 10 g/cm -sec for copper, theimpedances of the two materials are very well matched. An additionaladvantage provided by the utilization of copper damping means is thatthe damping means can be adapted to apply an electric field to the E-Ocrystals 31a, 31b and 31c along their respective C- axes. The polarityof the electric field applied to the E-O crystals is illustrated by theplus minus notations 40a, 40b and 400 in FIG. 3. The electrodes to 31a,31b and 310 are arranged so as to be electrically isolated from oneanother on thecrystal face.

FIG. 2 further illustrates acoustic vibration damping means forsuppressing the acoustic resonances occurring along the X axis withincomposite parallelepiped 30. As illustrated, acoustic vibration dampingmeans 38 and 39 take the form of thin planar sheets, and are positionedcontiguous oppositely disposed faces of composite crystal 30. It hasbeen found experimentally that the utilization of 3M acoustic dampeningtape such as Y9l62c or Y9273 is effective in reducing the inducedacoustic vibrations. The particular shape and composition of dampingmeans 38 and 39 is not critical so long as the acoustic energypropagating along the X axis is transmitted out of composite crystal 30and dampened or directed such that it does not re-enter the crystal.However, when damping means 34a, 34b, 35a, 35b, 36a and 36b are composedof a conducting material to allow application of an electric fieldthereby, damping means 38 and 39 must be composed of nonconductingmaterial to avoid short-circuiting the electric field.

Since the light beam enters and exits composite crystal 30 along the Yaxis, the damping means (not shown) utilized to dampen the acousticenergy occurring along the Y axis must be transparent to the incidentlight beam. For example, the faces of the composite crystal 30 throughwhich the light beam enters and exits can be cut at a Brewster angle forthe incident and exiting light beam. Such a technique is furtherdescribed in conjunction with the embodiment shown in FIG. 4.

Illustrated in FIG. 4 is a further embodiment of the present invention.A composite electro-optic crystal, generally designated 50, is comprisedof a plurality of polyhedrons 51a, 51b, 51c and 51d which are in theform of substantially identical right triangular prisms. The triangularprisms are positioned contiguous one another so as to form asubstantially square parallelepiped. In this embodiment the E0 crystalsare composed of potassium dihydrogen phosphate (KDP) type crystals.Acoustic vibration dampening means, generally designated 52, ispositioned contiguous the face of composite parallelepiped 50 formed byprism 51b. Dampening means 52 comprises acoustic impedance matchingmeans 56 and acoustic vibration absorbing means 57a and 57b. Likewise,acoustic vibration dampening means, generally designated 53, ispositioned contiguous the face of composite parallelepiped 50 formed bytriangular prism 51c. Dampening means 53 comprises acoustic impedancematching means 58 and acoustic vibration absorbing means 59a and 59b.

' Total internal reflection means 62 and 64 and mirrors 66 and 68 arepositioned about parallelepiped 50 so as to direct an incident lightbeam, generally designated 70, through the composite parallelepiped aplurality of times. Deflection of light beam 70 occurs as the beamtraverses the boundaries between the contiguous E-O crystals 51a, 51b,51c and 51d and is proportional to the magnitude of the electric fieldapplied along the C-axis of one or more of the crystals. To achieveadditional deflection, an additional mirror can be positioned adjacentE-O crystal 51d to return the exiting light beam back into the compositeprism.

The acoustic energy piezoelectrically induced along the X axis isillustratively shown in FIG. 4a as wavy lines 72 and 74. Since acousticimpedance matching means 58 is a material having an identical or nearlyidentical acoustic impedance as prism 51c, substantially all theacoustic energy incident the interface between prism 51c and impedancematching means 58 will enter 58. In the preferred embodimentillustrated, E-O crystals 51a, 51b, 51c and 15d are composed ofpotassium dideuterium phosphate (KD*P) and impedance matching means 56and 58 are composed of glass. The acoustic energy transmitted out ofcomposite parallelepiped 50 is then incident either face 60 or 61 ofacoustic impedance matching means 58. By cutting the acoustic impedancematching material so that the angle 6 is equal to the Brewster angle, alight beam incident face 60 or 61 is transmitted therethrough. Theacoustic energy, on the other hand, is reflected by either face 60 or 61toward acoustic vibration absorbing means 59a and 59b, respectively.Substantially all of the acoustic energy is then absorbed in theabsorbing means 59a and 59b. Acoustic damping means 52 functions in anequivalent manner to acoustic dampening means 53. Thus, the acousticenergy piezoelectrically induced along the X and Y axes is substantiallydampened thereby preventing the occurrence of acoustic resonances alongthese two axes within composite crystal 50. Utilizing KDP type E-Ocrystals, acoustic vibrations are not piezoelectrically induced alongthe C axis when the electric field is applied along the C axis. Thus, itis not necessary to provide damping means along this axis.

The present invention has been described in conjunction with a series ofpreferred embodiments. However, it will be obvious to one skilled in theart that modifications can be made to the embodiments described hereinwithout departing from the spirit or scope of the present invention. Forexample, the composition of the acoustic impedance matching means can beany composition which has nearly the same acoustic impedance as thecontiguous E-O crystal. Similarly, any geometrical configuration whichdoes not provide an acoustic energy feedback path to the contiguous E-Ocrystal can be utilized.

The embodiments of the invention in which an exclusive property or rightis claimed are defined as follows:

1. A light beam modulator comprising:

an electro-optic crystal for receiving an incident light beam and formodulating the beam propagating therethrough in response to an appliedelectric field by the electro-optic effect, the crystal havingpiezoelectrically induced acoustic vibrations in at least one directionwithin the crystal and having a plurality of faces, a first face of theplurality for receiving and transmitting an incident light beam, and

acoustic vibration damping means positioned contiguous at least secondand third substantially oppositely disposed faces of the plurality ofcrystalline faces and having substantially the same acoustic impedanceas the electrooptic crystal so that the acoustic vibrations incident theinterface between the crystal and the damping means are substantiallytransmitted into the damping means and dampened therein, wherein theacoustic vibration damping means form electrodes for applying anelectric field to the electro-optic crystal.

2. A light beam modulator comprising:

an electro-optic crystal for receiving an incident light beam and formodulating the beam propagating therethrough in response to an appliedelectric field by the electro-optic effect, the crystal havingpiezoelectrically induced acoustic vibrations in at least one directionwithin the crystal, and wherein the electro-optic crystal is formed by aplurality of adjacent polyhedrons, at least one of said polyhedronshaving an electric field dependent index of refraction, the plurality ofpolyhedrons comprising a first polyhedron in the form of a substantiallyisosceles triangular prism and second and third polyhedrons in the formof substantially identical right triangular prisms, the second and thirdpolyhedrons each being positioned contiguous a face of the firstpolyhedron so that the composite electro-optic crystal is in the form ofa rectangular parallelepiped having a plurality of faces, a first faceof the plurality for receiving an incident light beam, and acousticvibration damping means positioned contiguous two oppositely disposedfaces of each of the plurality of polyhedrons the acoustic vibrationdamping means being composed of an electrical conductive materialofsubstantially the same acoustic impedance as the adjacentelectro-optic crystal so that the acoustic vibrations incident theinterface between the crystal and the damping means are substantiallytransmitted into the damping means and dampened therein, the twooppositely disposed faces of each of the plurality of polyhedronsforming first and second oppositely disposed faces of the compositerectangular parallelepiped.

3. The light beam modulator of claim 2 wherein the acoustic vibrationdamping means includes further acoustic vibration damping means disposedcontiguous third and fourth oppositely disposed parallel faces of thecomposite rectangular parallelepiped.

4. A light beam modulator comprising:

an electro-optic crystal for receiving an incident light beam and formodulating the beam propagating therethrough in response to an appliedelectric field by the electro-optic effect, the crystal havingpiezoelectrically induced acoustic vibrations in at least one directionwithin the crystal, the electro-optic crystal being formed by aplurality of adjacent polyhedrons, at least one of said polyhedronshaving an electric field dependent index of refraction,

acoustic impedance matching means positioned contiguous at least oneface of each of the plurality of polyhedrons for transmitting theacoustic vibrations out of the contiguous electro-optic crystal into theacoustic impedance matching means, wherein the acoustic impedancematching means has at least one face thereof cut at the Brewster anglewith respect to the incident light beam so as to allow the light beam topropagate therethrough and to direct the acoustic vibrations incidentthereon toward acoustic vibration absorbing means, and

acoustic vibration absorbing means positioned contiguous the acousticvibration damping means for absorbing the acoustic vibrationstransmitted into the acoustic impedance matching means.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,63,091 I Dated May 16, 1972 Tzuo-Chang Lee Inventor(s) identified patentIt is certified that error appears in the aboveshown below:

and that said Letters Patent are hereby corrected as on the cover sheet[72] the inventor's name should read TzuoChang Lee Signed and sealedthis 7th day of November 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Attesting Officer ROBERT GOTTSCHALK Commissionerof Patents.

FORM Po-1050 (10-69) USCOMM-DC 60376-1 69 LLS GOVERNMENT PRINTING OFFICEI95 0365-3 4 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONPatent No. 3, 3,091 Dated May 16, 1972 Tzuo -Chang Lee Inventor(s) It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

On the cover sheet [72] the inventor-s name should read Two-Chang LeeSigned and sealed this 7th day of November 1972 (SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Atteeting Officer Commissionerof Patents FORM PO-105O (10-69) USCOMM DC 60376-P69 UTS GOVERNMENTPRINTING OFFICE I969 O366334,

1. A light beam modulator comprising: an electro-optic crystal for receiving an incident light beam and for modulating the beam propagating therethrough in response to an applied electric field by the electro-optic effect, the crystal having piezoelectrically induced acoustic vibrations in at least one direction within the crystal and having a plurality of faces, a first face of the plurality for receiving and transmitting an incident light beam, and acoustic vibration damping means positioned contiguous at least second and third substantially oppositely disposed faces of the plurality of crystalline faces and having substantially the same acoustic impedance as the electro-optic crystal so that the acoustic vibrations incident the interface between the crystal and the damping means are substantially transmitted into the damping means and dampened therein, wherein the acoustic vibration damping means form electrodes for applying an electric field to the electro-optic crystal.
 2. A light beam modulator comprising: an electro-optic crystal for receiving an incident light beam and for modulating the beam propagating therethrough in response to an applied electric field by the electro-optic effect, the crystal having piezoelectrically induced acoustic vibrations in at least one direction within the crystal, and wherein the electro-optic crystal is formed by a plurality of adjacent polyhedrons, at least one of said polyhedrons having an electric field dependent index of refraction, the plurality of polyhedrons comprising a first polyhedron in the form of a substantially isosceles triangular prism and second and third polyhedrons in the form of substantially identical right triangular prisms, the second and third polyhedrons each being positioned contiguous a face of the first polyhedron so that the composite electro-optic crystal is in the form of a rectangular parallelepiped having a plurality of faces, a first face of the plurality for receiving an incident light beam, and acoustic vibration damping means positioned contiguous two oppositely disposed faces of each of the plurality of polyhedrons, the acoustic vibration damping means being composed of an electrical conductive material of substantially the same acoustic impedance as the adjacent electro-optic crystal so that the acoustic vibrations incident the interface between the crystal and the damping means are substantially transmitted into the damping means and dampened therein, the two oppositely disposed faces of each of the plurality of polyhedrons forming first and second oppositely disposed faces of the composite rectangular parallelepiped.
 3. The light beam modulator of claim 2 wherein the acoustic vibration damping means includes further acoustic vibration damping means disposed contiguous third and fourth oppositely disposed parallel faces of the composite rectangular parallelepiped.
 4. A light beam modulator comprising: an electro-optic crystal for receiving an incident light beam and for modulating the beam propagating therethrough in response to an applied electric field by the electro-optic effect, the crystal having piezoelectrically induced acoustic vibrations in at least one direction within the crystal, the electro-optic crystal being formed by a plurality of adjacent polyhedrons, at least one of said polyhedrons having an electric field dependent index of refraction, acoustic impedance matching means positioned contiguous at least one face of each of the plurality of polyhedrons for transmitting the acoustic vibrations out of the contiguous electro-optic crystal into the acoustic impedance matching means, wherein the acoustic impedance matching means has at least one face thereof cut at the Brewster angle with respect to thE incident light beam so as to allow the light beam to propagate therethrough and to direct the acoustic vibrations incident thereon toward acoustic vibration absorbing means, and acoustic vibration absorbing means positioned contiguous the acoustic vibration damping means for absorbing the acoustic vibrations transmitted into the acoustic impedance matching means. 